This invention relates to catalysts for use in a reaction in which nitrogen oxides (NO.sub.x) in exhaust gases are selectively catalytically reduced with NH.sub.3.
Since photochemical smog is attributable to NO.sub.x released from power plants, sintering or firing ovens, various chemical plants, motor vehicles, etc., it has been desired to provide a method of effectively treating such pollutants. Among the processes heretofore proposed for denitrating exhaust gases, the process for catalytically reducing NO.sub.x with NH.sub.3 used as a reducing agent is considered advantageous in that the process can be practiced with a relatively small amount of reducing agent because NH.sub.3 selectively reacts with NO.sub.x even when the exhaust gas contains more than 1 vol. % of oxygen.
Catalysts already known for use in this process comprise a carrier such as activated alumina, silica-alumina or zeolite and a heavy metal compound deposited on the carrier. Such catalysts are generally granular and are used chiefly in the form of a fixed bed which is liable to be clogged up with the dust contained in exhaust gases or which involves a great pressure loss, thus giving rise to the necessity of using a blower of large capacity. These problems can be overcome to some extent by the use of a catalyst of increased particle or grain size, but the cores of catalyst particles will then fail to act effectively, resulting in a reduced efficiency. In view of the problems described, it appears favourable to use catalysts of honeycomb structure in avoiding the clogging of the catalyst layer with dust or the increase of pressure loss.
Power plants and sintering or firing furnaces usually give off large quantities of exhaust gases which require similarly large quantities of catalysts for treatment. Accordingly catalysts of honeycomb structure, if useful for this purposee, must be large-sized and have sufficient strength so as to be placeable into the treating unit free of any damage. Catalysts of honeycomb structure have already been proposed which comprise a honeycomb support of metal, ceramics or like refractory and an active catalytic component deposited on the support. However, a metal material, if used for the honeycomb structure, must be rendered porous over the surface through a cumbersome procedure so as to hold the active component thereon effectively, whereas structures of ceramics must have an increased wall thickness and be fired to sufficient hardness at a high temperature to retain the desired strength. Catalysts of this type therefore require much labor for the preparation of the honeycomb structure serving as a support for the active catalytic component and become inevitably expensive.